PROCESS FOR THE PRODUCTION OF A COATING ON THE SURFACE OF PLASTICIZING SCREWS FOR INJECTION MOLDING MACHINES

PROCEDE DE PRODUCTION D'UN REVETEMENT A LA SURFACE DE VUS DE PLASTIFICATION POUR PRESSES D'INJECTION

English Abstract

A process is disclosed for producing a coating on the surface of plastifier
screws for injection-moulding machines. A metallic, hard-metallic or ceramic
substance with a low distortion is applied to a steel screw base element by
high-speed flame spraying. During this coating process, a spray jet treats the
different surface regions of the screw base element (1) namely, rib top (3),
flanks (4) and screw base (5), partially independently.

French Abstract

L'invention a pour objet un procédé de production d'un revêtement à la surface de vis de plastification pour presses d'injection. Sur la vis constituée par un matériau de base en acier, on applique un matériau métallique, métallique dur ou céramique, à faible distorsion, par projection à la flamme à grande vitesse. Lors du processus de revêtement, un jet frappe les différentes zones de la surface du matériau de base de la vis (1), partiellement indépendamment les unes des autres, à savoir, la partie supérieure (3) et les flancs (4) des nervures, ainsi que la base (5) de la vis.

Claims

CLAIMS:

1. A process for producing a coating on a surface of a
screw for plasticizing in an injection molding machine in which
a metal, carbide-metal or ceramic material is applied as the
coating to a screw main body consisting of steel, by means of
at least one spray jet using a high-speed flame spray process
characterised in that in the coating operation the spray jet
acts separately on a top side of a flight of the screw, and on
flanks of the flight of the screw and on a bottom of the screw
the top, flanks and bottom being acted on by the spray jet
independently of each other, a mechanical bond being made
between the screw main body and the coating.
2. A process as set forth in claim 1 characterised in
that firstly the flight flanks and then the screw bottom and
the flight top side are coated.
3. A process as set forth in claim 1 characterised in
that edges of the screw flight are rounded.
4. A process as set forth in claim 3 characterised in
that radii at the rounded edges of the screw flights are
0.1- 0.8mm.
5. A process as set forth in claim 1 characterised in
that edges of the screw flight are provided with bevels.
6. A process as set forth in claim 5 characterised in
that the bevels at the edges of the screw flight are 0.1 to 3mm
wide and inclined to the top of the flight screw by an angle of
between 30 and 60 degrees.
7. A process as set forth in claim 1 characterised in
that the surface of the screw main body which is to be coated
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is cleaned of impurities in an ultrasonic bath prior to the
high speed flame spraying operation.
8. A process as set forth in claim 1 characterised in
that the surface of the screw main body which is to be coated
is cleaned of impurities manually by means of alcohol solutions
prior to the high-speed flame spraying operation.
9. A process as set forth in claim 8 characterised in
that a sand blasting procedure follows the cleaning procedure,
blasting agent used being iron-free electro-corundum in grain
sizes of 0.3-1.5mm.
10. A process as set forth in claim 1 characterised in
that the coating operation is thermally regulated so that the
maximum workpiece surface temperatures that occur are 150°C.
11. A process as set forth in any one of claims 1 to 10
characterised in that the coating is applied to the main body
of the screw by rotating the screw around its longitudinal axis
and by translating the spray jet parallel to the longitudinal
axis, the direction of rotation of the screw being changed at
reversal points for movements of the spray jet, the reversal
points being at the opposite ends of each flight of the screw.
12. A process as set forth in any one of claims 1 to 11
characterised in that for coating the flight flanks the spray
jet is guided a plurality of times axially and in opposite
directions over the screw main body, with the screw main body
rotating, wherein the forward feed movement of the spray jet
corresponds to a pitch of the screw flight and a direction of
rotation of the screw main body is changed at reversal points
of movement of the spray jet, which reversal points are at the
ends of the flight and after the operation of coating the
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flight flanks has been effected the screw bottom and the flight
top side are coated, wherein the spray jet is oriented at an
angle 90° relative to a longitudinal center line of the screw
main body.
13. A process as set forth in any one of claims 1 to 12
characterised in that for coating the flight flanks the spray
jet is guided a plurality of times axially and in oppositely
directed working stroke motions over the screw main body, with
the screw main body rotating, wherein the spray jet is moved
back in an idle condition to the starting point between two
working stroke motions and the direction of rotation of the
screw main body is maintained irrespective of the direction of
movement of the spray jet, and that after the operation of
coating the flight flanks has been effected the screw bottom
and the flight top side are coated, wherein the burner nozzle
is oriented at an angle of 90° relative to the longitudinal
center line of the screw main body.
14. A process as set forth in any one of claims 1 to 12
characterised in that for coating the flight top side, the
flight flanks and the screw bottom the spray jet is moved
axially over the screw main body, the direction of the spray
jet is changed at reversal points of its movement and the spray
jet and longitudinal center line of the screw main body make an
angle of between 15° and 75°.
15. A process as set forth in any one of claims 1 to 14
characterised in that the spray jet is focused by means of a
shield system and the flight top and the flight flanks are
coated such that linear motion of the spray jet is independent
of rotation of the screw.
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16. A process as set forth in any one of claims 1 to 15
characterised in that portions of the screw main body are
protected by means of silicone cover means from the spray jet
passing thereover.
17. A process as set forth in any one of claims 1 to 16
characterised in that one or more of the following materials is
or are used as the coating materials: Mo, Cr, Ni, Ti, Al, Co,
Nb, W, Co-Ni, Co-Cr, Cr-Ni, and Co-Mo-Cr.
18. A process as set forth in any one of claims 1 to 17
characterised in that the coating is applied in a layer
thickness of 0.1-0.8mm.
19. A process as set forth in any one of claims 1 to 18
characterised in that the coated surfaces of the screw main
body are grinded after the coating has been applied, where the
grinding is performed with tools of diamond or cubic boron
nitride.
20. A process as set forth in claim 19 characterised in
that the flight top side is ground with a grinding wheel and
the flight flanks and the screw bottom are ground with flexible
bands, wherein at least three different grinding grain sizes
are used.
21. A process as set forth in any one of claims 1 to 20
characterised in that the coated plasticizing screws are
subjected to torsional tests, wherein shear stresses of 180N/mm2
are induced at temperatures of 450° over at least 24 hours.
9

Description

CA 02206702 2000-07-31
23739-313
Process for the production of a coating on the surface of
plasticizing screws for injection molding machines
The invention relates to a process for producing a
coating on the surface of plasticizing screws for injection
molding machines, in which a metal, carbide-metal or ceramic
material is applied with a low degree of distortion to a screw
main body consisting of steel, by means of at least one spray
jet using the high-speed flame spray process.
In order to improve the wear resistance of
plasticizing screws, the conventional state of the art provides
that the plasticizing screws are hardened or coated by means of
flame spraying with subsequent heat treatment. Both can result
in deformation of the screw, while in the case of flame
spraying, cracks can occur in the coating in the subsequent
operation of straightening the plasticizing screw. A
plasticizing screw of that kind is therefore not resistant to
corrosion.
An object of the present invention is to provide an
improved process for coating plasticizing screws, by means of
which it is possible to produce a wear-resistant and corrosion-
resistant coating, which is operative to produce a conveyor
effect, on the surface of the plasticizing screw.
The invention provides a process for producing a
coating on a surface of a screw for plasticizing in an
injection molding machine in which a metal, carbide-metal or
ceramic material is applied as the coating to a screw main body
consisting of steel, by means of at least one spray jet using a
high-speed flame spray process characterised in that in the
coating operation the spray jet acts separately on a top side
of a flight of the screw, and on flanks of the flight of the
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CA 02206702 2000-07-31
23739-313
screw and on a bottom of the screw the top, flanks and bottom
being acted on by the spray jet independently of each other, a
mechanical bond being made between the screw main body and the
coating.
Therefore a fused bond is not made between the screw
main body and the coating.
If a plasticizing screw is coated with the process
according to the invention, there is no need for a subsequent
heat treatment which could result in distortion of the
plasticizing screw.
Various embodiments of the invention are described
hereinafter with reference to the Figures of the accompanying
drawings.
Figures 1 through 4 each diagrammatically show a part
of the screw main body and the spray jet with spray nozzle;
Figures 5 and 6 each show
la


CA 02206702 1997-06-03
~.,
a part of a screw main body with spray jet and spray nozzle, wherein two
different shield systems are shown above the spray jet, Figure 5a is a
view in section through the spray jet taken along line I-I in Figure 5
and Figure 7 shows a part of the screw main body and a cover means.
The screw main body 1 which is made of steel has a screw flight 2.
The screw flight 2 extends in a helical configuration around the screw
main body 1. It has a flight top side 3 and two flight flanks 4. Disposed
in the free space between the screw flight 2 is the screw bottom 5.
The pitch and/or configuration of the screw flight 2 varies over
the length of the screw main body 1. This configuration of the
plasticizing screw however is independent of the invention.
Coating of the entire screw surface 1 and the screw flight 2 is
- effected by means of a burner or torch nozzle 6 by means of which a spray
jet of metal, hard-metal or carbide-metal, or ceramic material is applied
- 15 to the screw main body 1 and the screw flight 2, using the high-speed
flame spray process.
In that procedure the burner nozzle 6 is moved axially a plurality
of times along the screw main body 1, with the screw main body 1 being
rotated about its longitudinal axis. In the example of the process shown
in Figures la through 1c the forward feed movement of the burner nozzle 6
corresponds to the pitch of the screw flight. The burner nozzle 6 is
oriented at an angle A of 0 - 60° relative to the line n normal to the
flight flank 4 of the screw flight 2.
At each reversal point the burner nozzle 6 is pivoted in order to
spray on to the oppositely disposed flight flank 4 of the screw flight 2,
and likewise at each reversal point the direction of rotation of the
screw main body 1 is changed.
After the flanks 4 of the screw flight 2 have been coated the screw
bottom 5 and the top side 3 of the flight is coated, in which case, as
shown in Figure 1c, the burner nozzle is oriented at an angle of 90°
relative to the longitudinal center line of the screw main body.
In the embodiment shown in Figures 2a through 2c the forward feed
movement of the burner nozzle 6 again corresponds to the pitch of the
screw flight and the burner nozzle 6 is again oriented at an angle of 0 -
60° relative to the line n normal to the flanks 4 of the screw flight.
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CA 02206702 1997-06-03
The screw main body 1 is rotated while the burner nozzle 6 is being
moved axially. At the reversal points the burner nozzle is turned, as
shown in Figures 2a and 2b. The direction of rotation of the screw main
body 1 however is retained.
The burner nozzle 6 is moved over the screw main body 1 with
working stroke motions in the same direction. Between two working stroke
motions the burner nozzle 6 moves back in an idle condition into the
starting position.
Coating of the screw bottom 5 and the top side 3 of the flight is
again effected by means of a burner nozzle 6 which is oriented
perpendicularly with respect to the longitudinal center line of the
screw main body 1.
In order to enhance the efficiency of coating on the flanks 4 of
the screw flight, it is possible to use shields 7, as shown in Figures 5
and 6. The shields 7 are preferably of a box-like configuration and are
fixed to the burner nozzle 6 or to a holder for the burner nozzle 6.
Disposed within the shield 7 is a suction removal device 8 which sucks
away the material which has passed into the shield 7.
The shields 7 are of a box-like configuration and have an inclined
bottom 9 at which there is an opening 10 through which coating material
can pass into the shield 7.
Advantageously the spacing between the bottom 9 of the shield 7 and
the top side 3 of the screw flight 2 is filled by a silicone seal 12.
The shield 7 is moved together with the burner nozzle 6 axially
along the screw flight 2 and is reversed with a pivotal movement
together with the nozzle 6 at the reversal points.
Figures 3 and 4 show a further alternative configuration in regard
to coating the entire screw main body 1. In that case the burner nozzle
6 is held at an angle of 15 - 75° with respect to the longitudinal
center line of the screw main body 1 and is again guided in the axial
direction over the surface of the screw main body 1 during the rotary
movement thereof. With that kind of coating operation the forward feed
movement of the burner nozzle 6 is independent of pitch.
During the reciprocating movement of the burner nozzle 6 the
direction of rotation of the screw main body can remain the same or it
may also be changed at each of the reversal points of the burner nozzle
6.
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CA 02206702 1997-06-03
In order to improve the adhesion of the coating material at the
edges 11 of the screw flight 2 it is advantageously provided that the
edges 11 are rounded or provided with bevels. The radii of rounded edges
11 are advantageously between 0.1 and 0.8 mm while the bevels are 0.1
through 3x30-60°.
Before the actual high-speed flame spraying operation the screw
main body 1 is advantageously cleaned in an ultrasonic bath or manually
by means of alcohol solutions. A sand blasting process follows the
cleaning procedure, iron-free electro-corundum of grain sizes of 0.3 -
1.5 mm being used as the blasting agent.
The coating operation is thermally regulated in such a way that the
maximum workpiece surface temperatures that occur are 150°C. In that
way
only minimal distortion of the screw main body 1 occurs and a
straightening process subsequent to the coating operation can be
omitted.
Particularly suitable materials for the screw main body 1 are metal
materials, preferably the materials 1.7225, 1.2316, 1.8550 and 1.8519.
In accordance with the process of the invention one or more of the
following materials are used as the coating materials: Mo, Cr, Ni, Ti,
A1, Co, Nb, W, Co-Ni, Co-Cr, Cr-Ni, Co-Mo-Cr, CrC-NiCr, WC-CrC-Ni, WC
Ni, TiC-CrNi, VC-CrNi, WC-CV-Ni, A10-ZrO, A10-TiO, Zr0-Mg0 and CrO.
The thickness of the coating layer is between 0.1 and 0.8 mm,
preferably between 0.2 and 0.4 mm.
Advantageously the coated surfaces are worked with diamond or CBN-
tools, wherein the outside fits of the screw flight 2 are ground with a
grinding wheel and the flight flanks 4 and the screw bottom 5 are ground
with flexible bands or strips. Preferably at least three different
grinding grain sizes are used.
The process according to the invention provides at the surface of
the screw for minimum adhesion in relation to plastic materials, an
optimum conveyor effect and a very good self-cleaning action for a wide
range of plastic materials (preferably Ra-values of between 0.10 and
0 . 4 0 ,um ) .
4


. . CA 02206702 1997-06-03
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The plasticizing screws which are coated with the process according
to the invention are subjected to torsional testing, in which case shear
stresses of 180 N/mm2 are induced at temperatures of 450°C over at
least
24 hours.
Instead of or in addition to the shield 7 it is also possible to
use a cover means as shown in Figure 7, which covers the screw flight 2
and parts of the screw bottom 5. Advantageously two shields 7 which are
spaced in the axial direction by the pitch of the screw flight 2 are
moved in parallel so that only one flight flank 4 is free.
The cover means 13 advantageously has a silicone layer 14 which is
carried by a carrier plate 15 of steel sheet. The cover means 13 is
guided over the screw main body 1 with the same forward feed movement as
the spray nozzle 6.
5

Representative Drawing